1. Field of the Invention
The present invention relates generally to antennas, and more particularly, to a log periodic dipole antenna having a microstrip feedline.
2. Description of the Prior Art
Although numerous varieties of log periodic antennas have been in widespread use for years, the log periodic dipole array is often favored because of its ability to operate over a broad frequency range. Because of its unique geometric arrangement, different elements in the array are active at different frequencies. As a result, the log periodic dipole antenna exhibits relatively constant operating characteristics, including gain, feed-point impedance and front-to-back ratio, over the frequency range supported by the log periodic dipole antenna.
The typical log periodic dipole antenna includes several dipole elements of varying lengths which are positioned and spaced according to length. The shortest elements are located at the feed end, or "front end", of the array, with each successive element being of equal or longer length. Also, the electrical connections of opposed elements are alternated to provide a phase shift of 180 degrees between elements.
Log periodic dipole antennas are almost universally fed by a balans feeder connected directly to the shortest elements at the front end of the array. A variety of feedlines are used including coaxial cables and external strip lines. However, these types of feeding arrangements have their shortcomings. First, antenna performance is derated by reduced impedance matching, power handling capacity and pattern interference. Moreover, these arrangements are cumbersome and make the feedline more susceptible to damage from weather elements such as wind and ice, especially when the antenna is mounted on a tall tower.
Consequently, an alternative arrangement for feeding a log periodic dipole antenna is highly desirable.